1. Field of the Invention
The present invention generally relates to an internal combustion engine having a hydraulic control system for controlling the operation of a variable camshaft timing (VCT) system of the type in which the position of the camshaft is circumferentially varied relative to the position of a crankshaft in reaction to engine oil pressure. In such a VCT system, an electro-hydraulic control system is provided to effect the repositioning of the camshaft and a locking system is provided to selectively permit or prevent the electro-hydraulic control system from effecting such repositioning.
More specifically, this invention relates to a multi-position VCT system actuated by engine oil pressure and having a large number of thin, spring-biased vanes defining alternating fluid chambers therein.
2. Description of the Prior Art
It is known that the performance of an internal combustion engine can be improved by the use of dual camshafts, one to operate the intake valves of the various cylinders of the engine and the other to operate the exhaust valves. Typically, one of such camshafts is driven by the crankshaft of the engine, through a sprocket and chain drive or a belt drive, and the other of such camshafts is driven by the first, through a second sprocket and chain drive or a second belt drive. Alternatively, both of the camshafts can be driven is by a single crankshaft-powered chain drive or belt drive. It is also known that the performance of an internal combustion engine having dual camshafts, or but a single camshaft, can be improved by changing the positional relationship of a camshaft relative to the crankshaft.
It is also known that engine performance in an engine having one or more camshafts can be improved, specifically in terms of idle quality, fuel economy, reduced emissions, or increased torque. For example, the camshaft can be xe2x80x9cretardedxe2x80x9d for delayed closing of intake valves at idle for stability purposes and at high engine speed for enhanced output. Likewise, the camshaft can be xe2x80x9cadvancedxe2x80x9d for premature closing of intake valves during mid-range operation to achieve higher volumetric efficiency with correspondingly higher levels of torque. In a dual-camshaft engine, retarding or advancing the camshaft is accomplished by changing the positional relationship of one of the camshafts, usually the camshaft that operates the intake valves of the engine, relative to the other camshaft and the crankshaft. Accordingly, retarding or advancing the camshaft varies the timing of the engine in terms of the operation of the intake valves relative to the exhaust valves, or in terms of the operation of the valves relative to the position of the crankshaft.
Heretofore, many VCT systems incorporated hydraulics including an oscillatable vane having opposed lobes and being secured to a camshaft within an enclosed housing. Such a VCT system often includes fluid circuits having check valves, a spool valve and springs, and electromechanical valves to transfer fluid within the housing from one side of a vane lobe to the other, or vice versa, to thereby oscillate the vane with respect to the housing in one direction or the other. Such oscillation is effective to advance or retard the position of the camshaft relative to the crankshaft. These VCT systems are typically xe2x80x9cself-poweredxe2x80x9d and have a hydraulic system actuated in response to torque pulses flowing through the camshaft.
Unfortunately, the above VCT systems may have several drawbacks. One drawback with such VCT systems is the requirement of the set of check valves and the spool valve. The check valves are necessary to prevent back flow of oil pressure during periods of torque pulses from the camshaft. The spool valve is necessary to redirect flow from one fluid chamber to another within the housing. Using these valves involves many expensive high precision parts that further necessitate expensive precision machining of the camshaft.
Additionally, these precision parts may be easily fouled or jammed by contamination inherent in hydraulic systems. Relatively large contamination particles often lodge between lands on the spool valve and lands on a valve housing to jam the valve and render the VCT inoperative. Likewise, relatively small contamination particles may lodge between the outer diameter of the check or spool valve and the inner diameter of the valve housing to similarly jam the valve. Such contamination problems are typically approached by targeting a xe2x80x9czero contaminationxe2x80x9d level in the engine or by strategically placing independent screen filters in the hydraulic circuitry of the engine. Such approaches are known to be relatively expensive and only moderately effective to reduce contamination.
Another problem with such VCT systems is the inability to properly control the position of the spool during the initial start-up phase of the engine. When the engine first starts, it takes several seconds for oil pressure to develop. During that time, the position of the spool valve is unknown. Because the system logic has no known quantity in terms of position with which to perform the necessary calculations, the control system is prevented from effectively controlling the spool valve position until the engine reaches normal operating speed. Finally, it has been discovered that this type of VCT system is not optimized for use with all engine styles and sizes. Larger, higher-torque engines such as V-8""s produce torque pulses sufficient to actuate the hydraulic system of such VCT systems. Regrettably however, smaller, lower-torque engines such as four and six cylinder""s may not produce torque pulses sufficient to actuate the VCT hydraulic system.
Other VCT systems incorporate system hydraulics including a hub having multiple circumferentially spaced vanes cooperating within an enclosed housing having multiple circumferentially opposed walls. The vanes and the walls cooperate to define multiple fluid chambers, and the vanes divide the chambers into first and second sections. For example Shirai et al., U.S. Pat. No. 4,858,572, teaches use of such a system for adjusting an angular phase difference between an engine crankshaft and an engine camshaft. Shirai et al. further teaches that the circumferentially opposed walls of the housing limit the circumferential travel of each of the vanes within each chamber.
Shirai et al. discloses fluid circuits having check valves, a spool valve and springs, and electromechanical valves to transfer fluid within the housing from the first section to the second section, or vice versa, to thereby oscillate the vanes and hub with respect to the housing in one direction or the other. Shirai et al. further discloses a first connecting means for locking the hub and housing together when each vane is in abutment with one of the circumferentially opposed walls of each chamber. A second connecting means is provided for locking the hub and housing together when each vane is in abutment with the other of the circumferentially opposed walls of each chamber. Such connecting means are effective to keep the camshaft position either fully advanced or fully retarded relative to the crankshaft.
Unfortunately, Shirai et al. has several shortcomings. First, the previously mentioned problems involved with using a spool valve and check valve configurations are applicable to Shirai et al. Second, this arrangement appears to be limited to a total of only 15 degrees of phase adjustment between crankshaft position and camshaft position. The more angle of cam rotation, the more opportunity for efficiency and performance gains. Thus, only 15 degrees of adjustment severely limits the efficiency and performance gains compared to other systems that typically achieve 30 degrees of cam rotation. Third, this arrangement is only a two-position configuration, being positionable only in either the fully advanced or fully retarded positions with no positioning in-between whatsoever. Likewise, this configuration limits the efficiency and performance gains compared to other systems that allow for continuously variable angular adjustment within the phase limits.
Therefore, what is needed is a VCT system that is designed to overcome the problems associated with prior art variable camshaft timing arrangements by providing a variable camshaft timing system that performs well with all engine styles and sizes, packages at least as tightly as prior art VCT hardware, eliminates the need for check valves and spool valves, provides for continuously variable camshaft to crankshaft phase adjustment within its operating limits, and provides substantially more than 15 degrees of phase adjustment between the crankshaft position and the camshaft position.
According to the present invention there is provided a Variable Camshaft Timing (VCT) system that is designed to overcome the problems associated with prior art variable camshaft timing arrangements. The present invention provides a variable camshaft timing system that performs well with all engine styles and sizes, packages at least as tightly as prior art VCT hardware, eliminates the need for check valves and spool valves, provides for continuously variable camshaft to crankshaft phase adjustment within its operating limits, and provides substantially more than 15 degrees of phase adjustment between the crankshaft position and the camshaft position.
In one form of the invention, there is provided a camshaft and a hub secured to the camshaft for rotation synchronous with the camshaft. A housing circumscribes the hub and is rotatable with the hub and the camshaft and is further oscillatable with respect to the hub and the camshaft within a predetermined angle of rotation. A plurality of driving vanes is radially disposed in the housing and cooperates with an external surface on the hub. Likewise, a plurality of driven vanes is radially disposed in the hub and cooperates with an internal surface of the housing. A locking arrangement reactive to oil pressure is provided for preventing relative motion between the housing and the hub at any of a multitude of circumferential positions of the housing and the hub relative to one another, Finally, a configuration for controlling the oscillation of the housing relative to the hub is provided.
Accordingly, it is an object of the present invention to provide an improved variable camshaft timing arrangement for an internal combustion engine.
It is another object to provide a variable camshaft timing arrangement in which the position of a camshaft is continuously variable relative to the position of the crankshaft within its operating limits.
It is still another object to provide a hydraulically operated variable camshaft timing arrangement of relatively simplified mechanical and hydraulic construction in contrast to an arrangement that requires check valves and spool valves.
It is yet another object to provide an improved VCT system that performs with all engine styles and sizes.
It is a further object to provide a VCT system that packages as tightly as previous VCT systems and eliminates the need for check valves and spool valves,
It is still a further object to provide a VCT that provides for continuously variable camshaft to crankshaft phase adjustment within its operating limits, and that provides at least approximately 30 degrees of phase adjustment between the crankshaft position and the camshaft position.
These objects and other features, aspects, and advantages of this invention will be more apparent after a reading of the following detailed description, appended claims, and accompanying drawings.